The present invention is directed to the field of digital image processing to produce half-tone image data for driving a printing machine, such as an image setter which images films or plates to be printed on a flexo-press (a flexographic press), and is also directed to the use of halftone image screens to produce enhanced image data from the data provided from digital sources including desktop publishing outputs, scanner outputs and digital camera outputs.
Digital halftoning is a computer graphics technique for producing an image, with many gray levels, on a binary imaging device in which the gray levels are approximated by black and white dots. The image presentation (character, shape, and shading) is achieved by changing percentage of area coverage (dot intensity) from region to region (pixel region to pixel region). Dot intensity is altered by altering dot size (amplitudexe2x80x94cluster dot pattern screening) or by altering dot density (frequencyxe2x80x94dispersed dot pattern screening).
In digital halftoning systems and methods, dispersed dot patterns are created by error diffusion techniques (stochastic generators) which provide a randomized grain effect pattern. The size of the halftone dot, whose shape can vary from screen format to screen format, is fixed for a given screen. Lighter and darker (gray) values are created by varying the population density of the dots. Cluster dot patterns are generated by selecting a distance between halftone dots for particular screen. Different gray values are achieved by varying the size of the dots. Conventional screen sets have been available with dot sizes that permit typically between 50 and 200 dots per inch.
Regardless of screen pattern selected, as pattern intensity is increased (percentage of area covered), image darkness and thereby resolution changes. With digital formats and digital implementation, an increase in gray value (number) results in an increased percent coverage.
In higher gray scale regions (darker) of an image generated, the cluster dot pattern screens have provided better visual acceptance of the imaging. In lower gray scale regions (lighter) of an image, the dispersed dot pattern screens have provided better visual acceptance of the imaging. It would therefore be logical to combine these two screens. However, when these two dot pattern screens have been combined to generate a single image, the transition between screen types has created an optically undesirable transition effect.
Moreover, in many reproduction processes, imaging dots below a specific size are not possible or problematic to generate and process. Therefore, reproducing gray levels in light tones is not possible or problematic, in such systems.
Flexographic press printing (relief printing with flexible rubber-like plates fastened to a cylinder) is a process in which the ability to achieve smaller imaged dot sizes has been limited. Typically, dot sizes have been limited to 40 microns or greater. Therefore, the definition quality of the printed image has been noticeably degraded in lighter areas.
What is desired is an improved digital imaging technique which provides enhanced digital imaging over the entire range of gray scale values, and specifically in low gray scale values.
What is also desired is an algorithm for implementing the digital technique which can enhance the image produced in lower gray levels (lighter areas of the image) when the smallest dot size available is not small enough to provide good images at those lower gray levels.
What is further desired is an apparatus to provide such enhanced digital imaging and for the implementation of the algorithm.
The invention provides a method of producing a halftone screen having a plurality of halftone dots arrayed along a desired screen frequency. The method comprises:
a) using variable size halftone dots arrayed along the screen frequency to reproduce shades of gray equal to and above a predetermined shade of gray, the predetermined shade of gray having a first halftone dot size; and
b) reproducing shades of gray below the predetermined shade of gray using the first halftone dot size arrayed along the screen frequency by deleting a number of halftone dots per unit area to obtain gray shades below the predetermined shade of gray.
The invention also provides a method for combining a halftone screen having a plurality of halftone dots of different size arrayed along a desired screen frequency with a halftone screen having a plurality of fixed size halftone dots arrayed at a random pattern without a visually objectionable transition area for reproducing shades of gray that are difficult to reproduce with either screen alone. The method comprises:
a) determining a minimum halftone dot size; and
b) reproducing a shade of gray requiring halftone dots smaller than the minimum dot size, by using a reduced number of the minimum size dots arrayed along the screen frequency.
Furthermore, the invention provides a method of manufacturing a halftone screen comprising:
a) selecting a minimum halftone dot size;
b) identifying a desired shade of gray requiring a halftone dot size smaller than the minimum halftone dot size for the screen;
c) calculating a number of halftone dots of the minimum size required to reproduce the desired shade of gray for the screen; and
d) replacing the smaller size halftone dots required to reproduce the shade of gray with the calculated number of minimum size halftone dots.